Electric motors have a wide field of application. In industrial production, for example, electric motors are used to drive pumps, conveyor belts, overhead cranes, fans, etc. An electric motor, adapted for use in a specific application, offers the user many advantages, mainly owing to its long life and limited need for maintenance. One basic requirement for a long electric motor life is that the rotor or stator in the electric motor does not have any faults or defects. Common types of rotor faults are, for example, breaks or cracks/fractures in a rotor bar, excessively high resistance in welded or soldered joints in the rotor, excessively large air cavities (as a result of the casting of the rotor) and rotor offset in air gaps relatively to the stator. Common types of stator faults are, for example, insulation faults between the turns of a winding, insulation faults between windings in the same phase, insulation faults between windings in different phases, insulation faults between windings and earth/motor casing, contaminated windings (i.e. impurities such as moisture, dust, or insulation charred due to overheating), an open turn of a winding in a delta-connected motor as well as contact problems between the winding ends and external connections.
When testing electric three-phase motors, it is common to measure current fundamental components during operation and to compare measurement data from the three phases. Usually, special sensors are used in these measurements to obtain measurement data.
It is known that it is possible to perform both on-line measurements and off-line measurements. Measuring methods carried out during operation (on-line measurements) are sensitive to disturbances in the power grid, i.e. fundamentals generated by other machines (for example switched power supply units, fluorescent tube fittings, etc.) that are connected to the same power grid. These disturbances cause erroneous measuring results and may even make measurements on the electric motor impossible.
When testing stators off-line according to the prior art, a powerful surge voltage with high energy content is supplied to the motor, following which the exponentially decaying response obtained is analysed to identify possible faults in the stator. This measuring method has many disadvantages, such as it may initiate or accelerate/bring to completion incipient insulation failures; it requires time-consuming and complex calculations and interpretations/analyses; it causes problems of pulse propagation in the winding due to L and C effects; it requires bulky and heavy equipment associated with transport/installation problems; and it is an expensive method. WO 2005/106514 discloses a method for safe checking of electric motors. This method discloses measuring a physical quantity, such as current (I), inductance (L) or impedance (Z), of the stator winding while the rotor being rotated about an rotation axis. Thereby, periodic measuring data relating to the physical quantity is obtained, and measuring data relating to at least two periods of the periodic measuring data is collected. For the majority of all three-phase asynchronous motors, a sinusoidal relationship between the rotor position and the physical quantity (I, L or Z) is present, being symmetric about the X-axis in each phase. According to the method, the symmetry between at least the fundamentals of two or more half-cycles of the collected measuring data is compared. Asymmetry in the measuring data indicates a rotor and/or stator fault.
When performing the method disclosed in WO 2005/106514, the rotor has to be rotated in fixed steps of equal size or by continuous rotation at a constant rate. If the rotor is not rotated in fixed steps or at constant rate, asymmetry in the measuring data occurs. Normally, this asymmetry would indicate a rotor/stator fault, but could also be due to non-continuous rotation. Therefore, it is important that the rotor is being rotated in fixed steps or by continuous rotation for obtaining a reliable result. Since it may be difficult under some circumstances to obtain a perfect rotation of the rotor, either by continuous rotation or at fixed steps, especially when rotating the rotor by hand, this requirement may be hard to fulfil for the above described technique under these circumstances.